Today on Galileo
Monday, October 15, 2001
The Io 32 Encounter Begins
Well, the preliminaries are out of the way, and today is the busiest on
Galileo's agenda for this encounter, number 32 in the series, with an
evening flyby of the volcanic satellite Io.
The day begins at 1:12 a.m. PDT [see note 1] with the Photopolarimeter
Radiometer instrument (PPR). The instrument turns its power on and performs
a calibration sequence to prepare for a full day of science observations.
Then at 1:29 a.m., the instrument turns its gaze on Jupiter for a
5-hour-long series of observations of the turbulent atmosphere of the giant
planet. These measurements blanket the northern polar region studying an
area populated by vortex-like storms. An additional observation at 4:42
a.m. collects data from the White Oval in the southern hemisphere. This
particular storm is the lingering result of a merger of several such storms
over the past few years, and has been the subject of repeated study by
Galileo's science instruments. Then at 8:01 a.m. PPR begins viewing Io,
mapping the temperatures of the surface of the dark side of the satellite.
At 9:11 a.m. the Fields and Particles suite of instruments begins a 2-hour
recorded measurement of a portion of Jupiter's magnetosphere near the Io
Torus known as the Ramp region. The torus is a doughnut-shaped area of
increased radiation and particle density that nearly coincides with the
orbit of Io. The Ramp is the transition between the background
magnetosphere and the torus. It is a region where the ion density and
temperature of the environment increase sharply, and that has not been
fully explored. The instruments participating in this observation are the
Heavy Ion Counter (HIC), the Energetic Particle Detector (EPD), the
Magnetometer (MAG), the Plasma Subsystem (PLS), and the Plasma Wave
At 10:58 a.m. the spacecraft enters the shadow of Jupiter, spending nearly
two hours in the dark as it continues its observations. The on-board
control software is configured to ignore the fact that the Sun can't be
seen. At 11:12 a.m. Galileo reaches its closest point to the largest of
Jupiter's satellites, Ganymede. But at over 1.4 million kilometers (890,000
miles) distance, it, like Callisto 3.5 hours later, is too far away to
present a tempting observation target.
At 11:58 a.m., an experiment conducted by the Radio Science Team begins.
This measurement takes advantage of the fact that the path of Galileo takes
it behind Jupiter as seen from Earth. As the radio signal sent from the
spacecraft appears to pass deeper and deeper into Jupiter's atmosphere, it
will be attenuated and refracted by the gases, and this will yield
information about electron density in the different layers of Jupiter's
ionosphere. Eventually, the atmosphere becomes too dense, and the signal
disappears altogether. This will occur at 12:23 p.m. At 2:09 p.m., the
signal will re-appear as the spacecraft passes out from behind the planet,
and the measurement sequence will be repeated in reverse as the occultation
ends. The Radio Science experiment ends at 3:41 p.m.
Even though the spacecraft is out of sight of Earth, science observations
can continue, as the data are placed on the on-board tape recorder for
later playback. Between 1 and 2 p.m., PPR again collects data for a thermal
map of Io's dark hemisphere.
The Solid State Imaging camera (SSI) next enters the picture, as it were,
at 2:24 p.m., snapping an image of Io with the giant volcano Loki appearing
on the limb. This will be our highest resolution look yet at this region of
Io. As we will be looking at the area from the side, we may get some
insight into the local topography, measuring relative heights of the features.
At 2:33 p.m. PPR captures an hour-long high-resolution temperature map of
the caldera, or volcanic crater named Colchis in Io's northern hemisphere.
This observation occurs while the feature is in the dark, and will measure
the intrinsic temperatures of the surface features, without being
complicated by reflected sunlight.
At 3:32 p.m. PPR performs another hour-long temperature map on Io that
covers the volcanic features Loki, Daedalus, and Amatarasu, in the northern
hemisphere. Then another hour's observation covers the Babbar Patera in the
southern hemisphere, followed by a half hour look at Ra Patera, also in the
While PPR is viewing the surface of Io, the Fields and Particles
instruments are recording continuously for the 2.5 hours. At this point in
the orbit, Galileo is passing through an expected "quiet" portion of the Io
Torus. The instruments will be measuring the electromagnetic and particle
interactions in the region, contributing to an understanding of particle
pickup processes near Io, and of thermal and non-thermal plasma interactions.
During this time as well, at 4:33 p.m. the spacecraft reaches its closest
point to Jupiter, at a distance of 4.8 Jupiter radii (343,000 kilometers or
213,000 miles) above the cloud-tops, slightly less than the distance from
the Earth to our Moon.
At 6:04 p.m. the Near Infrared Mapping Spectrometer (NIMS) enters the fray,
mapping the temperatures of the Pele volcano while that feature is in the
dark. This is followed at 6:10 p.m. by a PPR observation of the volcano
Loki and its environment. At 6:35 p.m. the focus on Loki continues with 10
minutes of observation by NIMS.
At 6:48 p.m. SSI shutters 7 images of the Pele volcano, which is in
darkness, looking for hot, glowing material to provide an estimate of the
temperatures in the region. These pictures will allow scientists to
distinguish features as small as 30 meters (98 feet) across, and will cover
more territory than similar pictures taken during our 24th orbit and flyby
of Io. NIMS follows on with a 6-minute temperature observation of the Pele
At 6:58 p.m. PPR looks straight down at Io as the spacecraft flies by, and
the instrument will measure temperatures of the swath of the satellite that
flows past its field of view for 10 minutes. During this observation, at
7:04:20 p.m., Galileo reaches its closest point to Io, at a distance of 181
kilometers (112 miles). At this time the spacecraft is traveling at 7.72
kilometers per second (17,270 miles per hour) relative to the satellite.
The latitude over which Galileo is flying is 78.6 degrees south, which is
equivalent to flying over Little America in Antarctica on Earth.
At 7:08 p.m. SSI provides a high-resolution look at the Telgonus Scarp
region, a cliff-like feature, looking for evidence of sapping or other
erosion mechanisms. This observation recovers a similar observation planned
on our 25th orbit in November 1999 which was lost when the spacecraft
experienced a fault and entered a safe state.
At 7:10 p.m NIMS performs a high-resolution thermal map of the southern
polar region of Io in order to map the distribution of sulfur dioxide.
At 7:14 p.m. SSI turns its attention to high-resolution images of lava
channels in the Emakong region, broader context images of the Telgonus
Scarp region previously viewed a scant 8 minutes earlier (we're flying away
from Io at a pretty good clip, now!), and a mountainous volcano named Tohil
captured near the terminator, or day-night boundary. This last viewing
geometry provides good shadows to determine relative heights of features.
At 7:23 p.m. NIMS spends 10 minutes mapping temperatures and sulfur dioxide
distribution around the Emakong caldera. Then both SSI and NIMS view an
active caldera named Tupan Patera, with SSI providing a 3-color image of
At 7:49 p.m. SSI collects a medium-resolution view of the Tvashtar volcano.
This site was the focus of activity during the last flyby in August, when
the trajectory of Galileo carried it nearly directly overhead of the
feature at close range. Unfortunately, at that time, electronic problems
with the camera prevented the return of any images near closest approach.
This will be the first medium-resolution look at the feature since the
large plume eruption late last year.
Two minutes later, NIMS views the Chaac caldera for thermal and sulfur
dioxide mapping for 10 minutes. Then SSI takes a medium-resolution (500
meters per pixel) picture of the Gish Bar region. With this picture we can
look for changes in the area since last seen in October 1999, on Galileo's
24th orbit of Jupiter. We will also be looking at a large apparent fracture
to the west of Gish Bar to determine its origin.
Just after 8 p.m. NIMS performs a 15-minute measurement of a new hot spot
in the northern hemisphere that was discovered on our last flyby in August.
As the current sequence of commands was already being developed during that
flyby, this observation represents a last-minute change to our viewing plan.
At 8:20 p.m. SSI views the terminator region of Io, which encompasses the
Mycenae region, Tohil Mons and Patera, Culann Patera, Zamama, Volund, and
several other poorly understood but intriguing sites. With the Sun low in
the sky over these features, longer shadows should provide scientists with
information about the relative heights of the features.
At 8:28 p.m. PPR returns to the observing plate with a scan along the
equator of Io, providing temperature data, followed by a map of a small
portion of Io's disk just south of the equator, this time looking at the
polarization of light reflected from the surface. This data will provide
information about the fine-scale structure of the surface materials.
At 8:54 p.m. NIMS begins an hour-long map from pole to pole of most of Io's
sunlit surface, mapping the locations of hot spots and sulfur dioxide
deposits. PPR follows at 10 p.m. with a 30-minute map focusing on the
northern latitudes of Io.
At 10:34 p.m. SSI snaps an image of the small inner satellite Amalthea,
with a view of the crater Pan on its surface. Current plans for Galileo
include flying within about 500 kilometers (310 miles) of this small moon
in November 2002.
At 10:36 p.m. PPR wraps up this very full day's observations with a
full-disk polarimetry map of the surface of Io, which takes us to midnight.
Now, you may be left with the feeling of "What's left to do?" Come back
tomorrow and see! We're not done yet!
Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time
(GMT). The time when an event occurs at the spacecraft is known as
Spacecraft Event Time (SCET). The time at which radio signals reach Earth
indicating that an event has occurred is known as Earth Received Time
(ERT). Currently, it takes Galileo's radio signals 41 minutes to travel
between the spacecraft and Earth. All times quoted above are in Earth